Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component

Five pentiptycene‐derived stilbene systems ( 1 R ; R =H, OM, NO, Pr, and Bu) have been prepared and investigated as light‐driven molecular brakes that have different‐sized brake components ( 1 H < 1 OM < 1 NO < 1 Pr < 1 Bu ). At room temperature (298 K), rotation of the pentiptycene rotor is fast (k_rot=10⁸–10⁹ s^?1) with little interaction with the brake component in the trans form ((E)‐ 1 R ), which corresponds to the brake‐off state. When the brake is turned on by photoisomerization to the cis form ((Z)‐ 1 R ), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)‐ 1 NO , (Z)‐ 1 Pr , and (Z)‐ 1 Bu , the rotation is nearly blocked (k_rot=2–6 s^?1) at 298 K. It is also demonstrated that the rotation is slower in [D₆]DMSO than in CD₂Cl₂. A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)‐ 1 H , (Z)‐ 1 OM , and (Z)‐ 1 NO , and it levels off on going from (Z)‐ 1 NO to (Z)‐ 1 Pr and (Z)‐ 1 Bu . DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E–Z photoswitching is up to 46 %, and both the E and Z isomers are stable under the irradiation conditions.     

Photoswitchable “Turn‐on” Fluorescence Diarylethenes: Substituent Effects on Photochemical Properties and Electrochromism

A series of “turn‐on” fluorescence diarylethenes derived from 2,3‐bis(2‐methylbenzo[b]thiophen‐3‐yl)‐5,6‐dihydro‐4H‐thieno[2,3‐b]thiopyran‐4‐one ( 1 ) with alkyl and acetyl substituents were synthesized. The photochemical and photophysical properties of these derivatives, including the photoreaction of crystalline 1 , were thoroughly investigated to reveal substituent effects on their properties. The results indicated that alkyl substituents did not significantly affect the absorption and emission spectra of the diarylethenes. However, large absorption and emission wavelength shifts were observed for the diarylethene with an acetyl substituent due to extension of π–π conjugation. Significantly, all of the fluorescent ring‐closed forms of the compounds isomerized to their ring‐open forms in the presence of Cu²⁺ in the dark. EPR results provide clear evidence for the formation of the compound 1 radical cation intermediate that might be generated in the reaction between c‐ 1 and Cu²⁺. DFT calculations found that the ground‐state activation energy for ring‐opening of 1^.+ was approximately 9.2 kcal mol^?1 lower than that of 1 without Cu²⁺, such that a Cu²⁺‐catalyzed oxidative cycloreversion reaction at room temperature might be possible.     

9,9‐Dimethylxanthene Derivatives with Room‐Temperature Phosphorescence: Substituent Effects and Emissive Properties

Room‐temperature phosphorescence (RTP) emitters with ultralong lifetimes are emerging as attractive targets because of their potential applications in bioimaging, security, and other areas. But their development is limited by ambiguous mechanisms and poor understanding of the correlation of the molecular structure and RTP properties. Herein, different substituents on the 9,9‐dimethylxanthene core (XCO) result in compounds with RTP lifetimes ranging from 52 to 601 ms, which are tunable by intermolecular interactions and molecular configurations. XCO‐PiCl shows the most persistent RTP because of its reduced steric bulk and multiple sites of the 1‐chloro‐2‐methylpropan‐2‐yl (PiCl) moiety for forming intermolecular interactions in the aggregated state. The substituent effects reported provide an efficient molecular design of organic RTP materials and establishes relationships among molecular structures, intermolecular interactions, and RTP properties.     

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.     

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical,Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid‐trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid‐cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD‐DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.     

Theoretical Investigation of the Additivity of Structural Substituent Effects in Benzene Derivatives

The additivity of substituent effects in 1,3- and 1,4-disubstituted C₆H₄X₂, and 1,3,5-trisubstituted C₆H₃X₃ (X=F, Cl, CN, NO₂, CH₃, CF₃, NH₂, OH) benzene derivatives on the ring geometry has been investigated. The analysis is based on ab initio calculations at the MP2/6-31G** level of theory. The substituent impacts on the benzene ring are generally in good agreement with the results reported in earlier experimental and lower level theoretical studies. The impacts determined in the monosubstituted benzenes were used to estimate the ring distortions in the di- and trisubstituted derivatives. The estimated ring CC bond distances agree generally within 0.001 Å and the estimated CCC bond angles within 0.3 degree, with the optimized ones. The best agreement (deviations up to only 0.0003 Å and 0.03 deg.) between the estimated and optimized geometrical parameters was obtained for the CH₃ derivatives. Generally, the para-disubstituted derivatives showed the best compliance with additivity, somewhat poorer agreement characterized the meta derivatives while the trisubstituted derivatives showed angular distortions of up to about 0.4°.     

Substituent Effects on the Ultraviolet Absorption Properties of 2,4-Dihydroxy Dibenzophenone

Substituent effects on the ultraviolet absorption properties of 2,4-dihydroxy dibenzophenone were investigated experimentally. Nine compounds of 2,4-dihydroxy dibenzophenone with different substituents were prepared by a solvent-free reaction of benzoyl chloride. The maximum absorption wavelength (λ_max) of these samples was measured, and their UV resistance properties in cotton fabric as well as in polyester were determined. The results show that the λ_max is dependent on the substituents at the benzylidene ring, and both electron donating substituents and electron withdrawing substituents cause a bathochromic shift. The UV resistance of fabric increases with the increase in compound concentration. The dyeing rate of each compound on polyester was higher than that of cotton. On cotton fabric, the dyeing rate of 2,4-dihydroxybenzophenone was the highest, 77.8%. On polyester, that of 2,4-dihydroxy-4′-ethyl dibenzophenone was the highest, 84.1%. The study provides new insights into the effect of substituents on the properties of 2,4-dihydroxy dibenzophenone that are related to the whitening of cotton and polyester materials.     

Synthesis and Self‐Association of Double‐Helical AADD Arrays

The design and syntheses of four self‐complementary oligomers that contain an underlying AADD hydrogen bond sequence are presented, and their self‐association was examined in the solution and solid state. The molecular recognition between the two strands is highly sensitive to substitutions of their component heterocycles. Substitution with electron‐donating and ‐withdrawing groups and the influence of preorganization has a large effect on the overall stabilities of the complexes studied. In particular, a wide range (>10⁵ M ^?1) of stabilities with respect to substitutions at various positions in the AADD oligomers was demonstrated. In the most extreme case, the dimerization constant measured (K_dimer≥4.5×10⁷ M ^?1) is comparable to the most stable homodimers of neutral AADD arrays reported to date.     

Protonation of Pyridyl‐Substituted TTF Derivatives: Substituent Effects in Solution and in the Proton–Electron Correlated Charge‐Transfer Complexes

Protonated pyridyl‐substituted tetrathiafulvalene electron‐donor molecules (PyH⁺‐TTF) showed significant changes in the electron‐donating ability and HOMO–LUMO energy gap compared to the neutral analogues and gave a unique N⁺?H???N hydrogen‐bonded (H‐bonded) dimer unit in the proton–electron correlated charge‐transfer (CT) complex crystals. We have evaluated these features from the viewpoint of the molecular structure of the PyH⁺‐TTF derivatives, that is, the substitution position of the Py group and/or the presence or absence of the ethylenedithio (EDT) group. Among 2‐PyH⁺‐TTF ( 1 o H⁺ ), 3‐PyH⁺‐TTF ( 1 m H⁺ ), 4‐PyH⁺‐TTF ( 1 p H⁺ ), and 4‐PyH⁺‐EDT‐TTF ( 2 p H⁺ ) systems, the para‐pyridyl‐substituted donors 1 p H⁺ and 2 p H⁺ exhibit more marked changes upon protonation in solution; a larger redshift in the intramolecular CT absorption band and a larger decrease in the electron‐donating ability. Furthermore, the EDT system 2 p H⁺ has the smallest intramolecular Coulombic repulsion energy. These differences are reasonably interpreted by considering the energy levels and distributions of the HOMO and LUMO obtained by quantum chemical calculations. Such substituent effects related to protonation were also examined by comparing the structure and properties of a new H‐bonded CT complex crystal based on 2 p H⁺ with those of its 1 p H⁺ analogue recently prepared by us: Both of them form a similar type of H‐bonded dimer unit, however, its charge distribution as well as the overall molecular arrangement, electronic structure, and conductivity were significantly modulated by the introduction of the EDT group. These results provide a new insight into the structural and electronic features of the PyH⁺‐TTF‐based proton–electron correlated molecular conductors.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

Dramatic Substituent Effects on the Photoluminescence of Boron Complexes of 2‐(Benzothiazol‐2‐yl)phenols

Substituents can induce dramatic changes in the photoluminescence properties of N,O‐chelated boron complexes. Specifically, the boron complexes of 2‐(benzothiazol‐2‐yl)phenols become bright deep blue‐ and orange‐red‐emitting materials depending on amino substituents at the 5‐ and 4‐positions of 2‐(benzothiazol‐2‐yl)phenol, respectively. Absorption and emission data show that the resulting boron complexes have little or small overlap between the absorption and emission spectra and, furthermore, X‐ray crystal structures for both the blue and orange‐red complexes indicate the absence of π–π stacking interaction in the crystal‐packing structures. These features endow the boron complexes with bright and strong photoluminescence in the solid state, which distinguishes itself from the typical boron complexes of dipyrromethenes (BODIPYs). A preliminary study indicates that the blue complexes have promising electro‐optical characteristics as dopant in an organic light‐emitting diode (OLED) device and show chromaticity close to an ideal deep blue. The substituent effects on the photoluminescent properties may be used to tune the desired emission wavelength of related boron or other metal complexes.     

Comparative Study of Photochromic Ferrocene‐Conjugated Dimethyldihydropyrene Derivatives

The photochemical properties and the mixed‐valence state of bis(ferrocenylethynyl)benzodimethyldihydropyrene ( 1 ) and other benzodimethyldihydropyrene (BzDHP) derivatives were investigated to understand the reversible photoswitching in the electronic communication of 1 . Absorption spectra of 1 were characterized by UV/Vis spectroscopy and calculated by using time‐dependent density functional theory (TD‐DFT), and the d orbitals of the ferrocene (Fc) moieties were shown to contribute to the occupied valence orbitals that were responsible for the photochromic behavior. 1 exhibited reversible photoisomerization in THF; however, photochromic behavior was not observed in dichloromethane. Analysis of redox potentials showed that the mixed‐valence state of 1 was more stable in dichloromethane than in THF. This is consistent with the observation that chemical oxidation led to an intervalence charge‐transfer (IVCT) band between the Fc moieties in the mixed‐valence state of 1 in dichloromethane, whereas such a band was not observed for one‐electron‐oxidized 1 in THF. Bis(pentamethylferrocenylethynyl)benzodimethyldihydropyrene ( 2 ) did not show photochromic behavior even in THF. The mixed‐valence state of 2 was much less stable than that of 1 in dichloromethane, and no obvious IVCT band was observed for one‐electron‐oxidized 2 in dichloromethane. The difference in the redox contribution of Fc and pentamethylferrocene (Me₅Fc) to BzDHP played an important role for these redox and photochromic behaviors; this was supported by analysis of valence orbital energies from DFT calculations. Designing molecules that connect redox centers through the use of a photochromic linker with a redox potential close to that of the redox centers could constitute a useful approach for the production of photochromic redox‐active metal complexes with strong electronic communication.     

2,7‐Di‐tert‐butylnaphtho[1,8‐cd][1,2]dithiole 1,2‐dioxides: Thermally Stable,Photochemically Active vic‐Disulfoxides

Bulking up: The thermal barrier to rearrangement of a vic‐disulfoxide is significantly increased through steric buttressing about the (O)S? S(O) bond. Whereas the title compounds represent the most thermally stable vic‐disulfoxides known to date, they also undergo a novel photomediated epimerization at room temperature (see scheme).

     

Functionalization of Hexa‐peri‐hexabenzocoronenes: Investigation of the Substituent Effects on a Superbenzene

We have demonstrated that the iridium‐catalyzed direct borylation of hexa‐peri‐hexabenzocoronene (HBC) enables regioselective introduction of boryl groups to the para‐, ortho‐, and meta‐substituted HBCs in high yields. The boryl groups have been transformed into various functionalities such as hydroxy, cyano, ethynyl, and amino groups. We have elucidated that the substituents significantly influence the photophysical properties of HBCs to enhance fluorescence quantum yields. DFT calculations revealed that the origin of the substituent effect is the lift in degeneracy in the frontier orbitals by an interaction with electron‐donating and electron‐withdrawing substituents at the para‐ and ortho‐positions. The change in molecular orbitals results in an increase of the transition probability from the S₀→S₁ states. In addition, the two‐photon absorption cross‐section values of para‐substituted HBCs are significantly larger than those of ortho‐ and meta‐substituted HBCs.     

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.